The electrode material (contact material) used for electrodes of vacuum interrupters (VI), etc. is required to satisfy characteristics, such as (1) the breaking capacity being large, (2) the withstand voltage capability being high, (3) the contact resistance being low, (4) the deposition resistance property being high, (5) the contact consumption being low, (6) the chopped current being low, (7) the workability being excellent, and (8) the mechanical strength being high.
Since some of these characteristics conflict with each other, there is no contact material satisfying all of the above characteristics. Cu—Cr electrode materials have characteristics, such as the breaking capacity being large, the withstand voltage capability being high, and the deposition resistance property being high. Therefore, they are widely used as contact materials of vacuum interrupters. Furthermore, there is a report that, in Cu—Cr electrode materials, one having a finer particle size of Cr particles is fine in breaking current and contact resistance (for example, Non-patent Publication 1).
In recent years, there has been progress in making vacuum interrupters conducting arc extinction of vacuum circuit breakers have smaller sizes and larger capacities. Thus, there has been an increasing demand for Cu—Cr based contact materials having withstand voltage capabilities superior to those of conventional Cu—Cr electrode materials, which are essential for making vacuum interrupters have smaller sizes.
For example, in Patent Publication 1, there is described a method to for manufacturing an electrode material, in which, as a Cu—Cr based electrode material excellent in electrical characteristics such as current breaking capability and withstand voltage capability, respective powders of Cu used as a base material, Cr for improving electrical characteristics, and a heat-resistant element (molybdenum (Mo), tungsten (W), niobium (Nb), tantalum (Ta), vanadium (V), zirconium (Zr), etc.) for making the Cr particles finer are mixed together, and then the mixed powder is put into a mold, followed by pressure forming and making a sintered body.
Specifically, a heat-resistant element is added to a Cu—Cr based electrode material containing as a raw material a Cr having a particle size of 200-300 μm, and the Cr is made fine through a fine texture technology. That is, an alloying process of the Cr element and the heat-resistant element is accelerated, and the precipitation of fine Cr—X (X is a heat-resistant element) particles in the inside of the Cu base material texture is increased. As a result, the Cr particles having a diameter of 20-60 μm in a configuration to have the heat-resistant element in its inside are uniformly dispersed in the Cu base material texture. Furthermore, there is also known a method for producing an electrode material (for example, Patent Publication 2) in which, without going through the fine texture technology, a powder obtained by mixing and pulverizing a reaction product of Cr and a heat-resistant element is mixed with a Cu powder, followed by pressure forming and then sintering to manufacture an electrode material containing Cr and the heat-resistant element in the electrode texture similar to Patent Publication 1.
By forming an arc-resistant metal's fine dispersion texture as described in Patent Publication 2, withstand voltage capability and breaking capability are improved, but deposition resistance property may be impaired. An inferior deposition resistance property causes a deposition between the electrodes when applying a large current in a closed condition of the electrodes. This lowering of deposition resistance property causes vacuum circuit breakers to have larger sizes, and this has been a task for mass-production.
Thus, it has been tried to manufacture an electrode material having superior withstand voltage capability and deposition resistance property by adding a low melting metal (e.g., tellurium (Te), etc.) to an electrode material having a MoCr fine dispersion texture (e.g., Patent Publications 3 and 4).
However, in the sintering step of a MoCr fine dispersion electrode material containing a low melting metal added thereto, there was a risk that vacancies were generated in the electrode interior to result in lowering of packing percentage of the electrode material. If packing percentage of the electrode material lowers by the generation of vacancies in the electrode material, there is a risk that brazing material (e.g., Ag) is absorbed into vacancies of the electrode's inside in the brazing step to result in difficulty in brazing of the electrode material.
Furthermore, Te is known to be higher in toxicity, as compared with bismuth (Bi) as a low melting metal similar to Te. For example, oral-mouse-LD50 (median lethal dose) of Te is 20 mg/kg (cited from Material Safety Data Sheet (MSDS) of KOJUNDO CHEMICAL LABORATORY CO., LTD). In contrast, oral-mouse-LD50 of Bi is 5000 mg/mg or greater (cited from Material Safety Data Sheet (MSDS) of Metali Co., Ltd.). LD50 of Te is in excess of 200 times LD50 of Bi. If Te is included in controlled substances of foreign REACH (Registration, Evaluation, Authorization and Restriction of Chemicals: REACH regulations) etc., its export limitation is speculated.